Can your metabolism ever be too fast?
Yes, if you have a medical condition like hyperthyroidism, or if you're addicted to a stimulant drug such as cocaine or methamphetamine (hopefully not the case!). Only about 2% of American women are actually considered underweight, according to the Centers for Disease Control and Prevention, but if you find that you're unexpectedly losing weight or having trouble keeping it on, see your doc to rule out an underlying disease.

Why do I seem to gain weight in the winter?
You might see seasonal changes in the number on the scale, but it doesn't have anything to do with the weather: "People are likely to be more active in spring and summer," explains Pamela Peeke, MD, assistant professor at the University of Maryland and author of The Hunger Fix. "There's a slight slowdown in your thyroid in the fall, but not enough to matter," she adds. "People who are active notice nothing."

When skinny models and celebs shrug and say, "I just have a fast metabolism!" is that really true?
It could be. "They may simply be genetically blessed," says Dr. Peeke (lucky them). "Then again, many are actually not eating that much in general and/or they're exercising a lot."